feat(notifications): add Phase 4 Slice 2 — multi-channel notification system

Full notification infrastructure with in-app, email, Slack, and Teams channels:

Backend:
- NotificationConfig, NotificationLog, Notification models + migration
- Notification service with event routing, channel delivery, retry logic
- 9 API endpoints (config CRUD + in-app notifications)
- APScheduler retry job with exponential backoff (30s, 2m, 10m)
- Wired into escalation, proposal approval, and knowledge flywheel
- Pydantic event key validation, cross-tenant protection on recipients

Frontend:
- TypeScript types + API client for all notification endpoints
- NotificationsPanel: bell icon with unread badge, dropdown, mark-read
- NotificationSettings: channel config, event toggles, test, delete confirm
- Notifications tab on IntegrationsPage
- ARIA attributes, Escape handler, settings link on panel

Review fixes (13 issues resolved):
- notify() no longer commits/rolls back caller's transaction (critical)
- retry_failed_notifications returns count instead of None (critical)
- NotificationSettings moved inside dedicated tab (critical)
- target_user_ids scoped by account_id (security)
- Email loop collects all failures before raising
- Slack webhook validates response body
- events_enabled rejects unknown event keys
- link column widened to String(500)
- Dead code removed from _auto_reinforce
- Delete confirmation, ARIA, Escape key support

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/index.ts

@@ -27,3 +27,4 @@ export { sessionToFlowApi } from './sessionToFlow'
 export { aiSessionsApi } from './aiSessions'
 export { flowProposalsApi } from './flowProposals'
 export { flowpilotAnalyticsApi } from './flowpilotAnalytics'
+export { notificationsApi } from './notifications'

frontend/src/api/notifications.ts

@@ -0,0 +1,57 @@
+import apiClient from './client'
+import type {
+  NotificationConfig,
+  NotificationConfigCreate,
+  NotificationConfigUpdate,
+  AppNotification,
+  UnreadCount,
+} from '@/types/notification'
+
+export const notificationsApi = {
+  async listConfigs(): Promise<NotificationConfig[]> {
+    const response = await apiClient.get<NotificationConfig[]>('/notifications/configs')
+    return response.data
+  },
+
+  async createConfig(data: NotificationConfigCreate): Promise<NotificationConfig> {
+    const response = await apiClient.post<NotificationConfig>('/notifications/configs', data)
+    return response.data
+  },
+
+  async updateConfig(id: string, data: NotificationConfigUpdate): Promise<NotificationConfig> {
+    const response = await apiClient.patch<NotificationConfig>(`/notifications/configs/${id}`, data)
+    return response.data
+  },
+
+  async deleteConfig(id: string): Promise<void> {
+    await apiClient.delete(`/notifications/configs/${id}`)
+  },
+
+  async testConfig(configId: string): Promise<{ success: boolean; message: string }> {
+    const response = await apiClient.post<{ success: boolean; message: string }>(
+      '/notifications/configs/test',
+      { config_id: configId }
+    )
+    return response.data
+  },
+
+  async list(params?: { skip?: number; limit?: number }): Promise<AppNotification[]> {
+    const response = await apiClient.get<AppNotification[]>('/notifications', { params })
+    return response.data
+  },
+
+  async unreadCount(): Promise<number> {
+    const response = await apiClient.get<UnreadCount>('/notifications/unread-count')
+    return response.data.count
+  },
+
+  async markRead(id: string): Promise<void> {
+    await apiClient.patch(`/notifications/${id}/read`)
+  },
+
+  async markAllRead(): Promise<void> {
+    await apiClient.post('/notifications/mark-all-read')
+  },
+}
+
+export default notificationsApi